Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
Boilers.ca is a brand of ROMA Heating & Cooling · Licensed gas contractor LGA0202903
Comparison
Verdict in brief
The KBR 21-3A and KBR 28-3A are the two smallest floor-standing Greenstar boilers: both weigh 150 lb in the 557 x 1048 x 555 mm FS cabinet, fire down to 24,600 BTU/hr, rate 33 dB(A) and reach 190 °F supply at 30 psi. The 28-3A's maximum input rises to 100,000 BTU/hr from 79,200 BTU/hr — a heat-loss calculation separates them.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Bosch Greenstar KBR 21-3A | Bosch Greenstar KBR 28-3A |
|---|---|---|
| Input rate, minimum | 24.6 MBH | 24.6 MBH |
| Input rate, maximum | 79.2 MBH | 100 MBH |
| Output rate, maximum (104/86 °F) | 74.7 MBH | 93.8 MBH |
| Weight (without packaging) | 150 lb | 150 lb |
| Sound pressure level | ≤ 33 dB(A) | ≤ 33 dB(A) |
| Maximum supply temperature | 190 °F | 190 °F |
| Maximum permissible operating pressure (PMS), heating | 30 psi | 30 psi |
Both models are heating-only floor boilers documented in the Greenstar FS/Combi FS I&O manual (6720810590), sharing the 21-15/16 x 41-1/4 x 21-7/8 in cabinet, the 150 lb weight, the 33 dB(A) sound rating, 190 °F maximum supply temperature and 30 psi heating pressure.
Minimum fire is the same 24,600 BTU/hr, and the only capacity difference sits at the top of the range: the KBR 28-3A prints 100,000 BTU/hr maximum input against the KBR 21-3A's 79,200 BTU/hr, with maximum output at 104/86 °F of 93,800 against 74,700 BTU/hr.
Both vent in the 21/28 class of the FS tables (52 ft of straight 2" concentric with two elbows on NG), so venting does not separate them. Note the FS manual prints 100,000 BTU/hr (29.3 kW) for the KBR 28-3A, while the Greenstar Wall manual prints 100,800 BTU/hr (29.5 kW) for the equivalent ZBR 28-3A — an edition-to-edition difference, not a product difference.
Bosch Thermotechnology
The Greenstar KBR 21-3A is a floor-standing heating-only Greenstar boiler: 24.6 to 79.2 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Bosch Thermotechnology
The Greenstar KBR 28-3A is a floor-standing heating-only Greenstar boiler: 24.6 to 100 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Structurally yes: same 150 lb FS cabinet, 33 dB(A), 190 °F / 30 psi ratings and 24,600 BTU/hr minimum fire. Only the maximum input differs — 79,200 against 100,000 BTU/hr.
The heating figures mirror the wall line, but the FS cabinet is floor-standing, weighs 150 lb and is rated 190 °F maximum supply in the FS manual (187 °F in the Wall manual).
The one that matches the heat-loss calculation: below about 79 MBH the KBR 21-3A suffices, and the KBR 28-3A only adds headroom without changing the footprint.
A consultation includes a heat-loss review so the size is right the first time.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.